The present invention generally relates to a photometer for measuring atomic fluorescence and, in particular, relates to such a photometer having means for compensating for lamp or window errors.
A conventional photometer is described in the German Offenlegungsschrift No. 30 01 053. In this prior art arrangement, a heated dissociation device is connected to the measuring cell, in which dissociation device volatile hydrides of a sought element which are gained from a sample solution by adding reagent, are flamelessly thermically decomposed. The free atoms thus developing in the heated dissociation device enter the measuring cell and are excited to atomic fluorescence by an exciting light beam. The measuring cell is substantially cubic and includes a window for the exciting light beam to enter as well as two windows on diametrically opposite sides. The fluorescence radiation is detected by means of a photoelectric detector through one window. In general, a concave mirror is disposed in front of the opposite window, which concave mirror reflects the fluorescence radiation emerged into this direction back to the detector.
Preferably, spectral lamps having electrodeless discharge are used for exciting atomic fluorescence. These spectral lamps suffer from the disadvantage that the radiation density thereof is often unstable or stabilizes only after a long working period. A further problem exists in that the windows of the measuring cell are often contaminated by decomposition products of the sample. This also results in errors.
Double-beam spectrophotometers are known in which a measuring and a reference light beam are alternately directed to a photoelectric detector by a suitable chopper. The measuring light beam passes through a measuring cell and the reference light beam passes through a reference cell which may also contain pure solvent, for example. The influence of variation of lamp brightness and the influence of the cell can be compensated for by processing the electric signals detected at the detector.
In the prior art double-beam spectrophotometers, a measuring and reference light beam emanating from a light source immediately impinged upon the photoelectric detector. These prior art structures are therefore not suitable for photometers measuring atomic fluorescence. Particularly, it would not be possible using the techniques of usual doublebeam spectrophotometers to compensate for errors resulting from contamination of the windows of the measuring cell by decomposition products of the sample.